This study involves a computer simulation of the glomerulus peritubular capillaries interstitial space and proximal tubule cells. Appropriate one dimensional equations have been writTen for convective and diffusive flow in each of these structures and are being solved numerically. The final objective is to synthesize a complete system simulation capable of reproducing the known behavior of the proximal tubule under a variety of experimental circumstances. The glomerular and peritubular capillary models have been completed. The third phase of this project consists of modelling fluid flow in intercellular spaces which communicate as shunt pathways between the interstitial space and the lumen of proximal tubule. It is known that metabolism drives active sodium chloride transport, from lumen to interstitial space, and that the flow of salt generated by this process provides an osmotic effect which leads to the reabsorption of water. In addition, it has been shown that the hydrostatic pressure and colloid osmotic pressure in pertubular capillaries can influence the rate of salt and water reabsorption. This has suggested that a hydrostatic pressure dependent pathway exists in parallel with the active transport pathway, and anatomical considerations suggest that the intercellular channels bounded on the lumenal end by so-called tight junctions, provide the physical pathway for these flows. Kinetic equations of motion have been written to describe flow through the intercellular channel, and phenomenological equations of irreversible thermodynamics have been formulated to describe flow across its boundaries. The system of equations provides a nonlinear two-point boundary value problem with nonlinear boundary conditions, and we are currently at work attempting to solve this problem.